Much hay, straw and forage contains dust of vegetable, mineral and most significantly biological particles, such as moulds and bacteria (pathogens). A proportion of these particles becomes airborne when the forage is disturbed, including when it is eaten, for example when a horse pulls a mouthful of hay from a rack.
Many horses and ponies become allergic to these dusts, which cause respiratory problems affecting their performance and frequently preventing them from work. Permanent lung damage may be caused, and the value of the animal may be reduced. Grooms and other attendants can be similarly affected with a complaint known as farmer's lung.